Greater than 50% of pregnant women take at least one prescription drug. Drug selection, dosing, and monitoring are important for all patients. However, the intricacies of physiological changes during pregnancy and the implications of drug therapy on the health and well-being of pregnant mothers and the developing fetus complicate treatments during gestation. Clinical evidence informs us that pregnancy alters hepatic drug metabolism, but the causative factors responsible for this phenomenon remain to be identified. Pregnancy increases elimination of cytochrome P450 (CYP)2A6, -2C9, -2D6, or -3A4 substrate drugs, while decreasing elimination of CYP1A2 or -2C19 substrates. Of the physiological changes associated with pregnancy, most pronounced is a dramatic increase in the production of female hormones, i.e., estrogens and progesterone (PRG). Preliminary results from our laboratory indicate that estradiol (E2) modulates expression of CYP1A2 and -2A6 whereas PRG upregulates CYP2A6 and -3A4 in human hepatocytes. For these CYP enzymes, the directional changes in their expression are similar to the clinically reported pharmacokinetic changes, suggesting that female hormones may be in part responsible for the changes in drug metabolism during pregnancy. Of note, E2 and PRG do not recapitulate all the changes in drug metabolism during pregnancy, such as induction of CYP2D6 activity. This indicates that as yet unknown factors regulate CYP expression during pregnancy. Preliminary data from our laboratory indicate presence of such factors in pregnant women's plasma. The central hypothesis of this project is that physiological changes accompanying pregnancy modulate CYP expression, leading to altered drug metabolism. The following specific aims are proposed: (1) Characterize combined effects of female hormones on CYP expression and drug metabolism. (2) Elucidate regulatory mechanisms for CYP3A4 induction by PRG. (3) Identify regulatory mechanisms for CYP2D6 induction during pregnancy. (4) Establish a model system to study altered drug metabolism during pregnancy, using human hepatocytes incubated in pregnant women's plasma. The proposed studies will enable us to identify and characterize pregnancy-relevant factors that are responsible for altered drug metabolism during pregnancy, establish their effects on CYP expression, and determine the underlying regulatory mechanisms for two clinically most important CYP enzymes, CYP2D6 and CYP3A4. The complex physiological changes during pregnancy and the consequences of drug exposure to mothers and their developing fetuses underscore the importance of elucidating potential mechanisms for the regulation of drug metabolism during pregnancy. Our preliminary work in this area and the studies proposed herein will ultimately help us accurately predict pharmacokinetic changes of drugs in pregnant women. This information will have a valuable impact on drug selection and dosing during pregnancy.